1. Field of the Invention
This invention concerns a porous fibrous structure and, more in particular, it relates to a novel porous fibrous structure having a plurality of indefinite cellular cavities inside the fibrous structure and a manufacturing method thereof.
2. Description of the Prior Art
Products prepared by bonding non-woven fabrics or block form fibrous structures with adhesives or adhesive fibers have been generally used. Such products are required for bulkiness depending on the application uses, for which various proposals have been made. For example, a method of impregnating a fibrous structure with a foaming agent to provide bulkiness by foaming of the agent and a method of dissolving water soluble fibers out of the fibrous structure containing the water soluble fibers thereby providing bulkiness have been proposed. Although such prior art techniques provide bulkiness to some extent to the fibrous structure, their effect is extremely small.
Further, in the former method, since the fibers are embedded in the foamed resin, property inherent to the fibers such as softness and hygroscopic property can not be developed. In the latter method, the size of the cavity formed between the fibers is not larger than that of the fibers removed by dissolution and, as a result, the porosity is restricted and the utilization factor of the fibers is reduced since the starting fibers are partially removed by dissolution.
Further, Japanese Published Unexamined Patent Application Sho 59-76959 and Sho 60-28565 disclose non-woven fabrics obtained by mixing polypropylene fibers formed by incorporating azodicarboxylic acid amide as a blowing agent into the surface of the fibers thereby forming a plurality of fine cleft holes to the surface with binder fibers and applying heat treatment. However, the non-woven fabric obtained by this method has no cellular cavities in the non-woven fabric but this intends to modify the surface smoothness of the synthetic fibers by the presence of fine cleft holes on the surface of the fibers. On the other hand, wadding is applied as filling fibers or core materials of mattresses and stuffed dolls but any of them has uniform structure in which wadding is entangled at random and has no cellular cavities.
Fibrous structures consisting only of fibers and with extremely high porosity having indefinite cellular cavities have not yet been known.
This invention intends to provide a novel fibrous structure having an entangled structure of fibers and cellular cavities together, as well as a manufacturing method thereof.
The foregoing object can be attained in accordance with this invention in a porous fibrous structure containing from 10 to 100% by weight of wet heat bonding fibers, in which a plurality of indefinite cellular cavities are present independently or in a state that a plurality of them are connected partially in the inside of the fibrous structure, and at least a portion of the fibers constituting the fibrous structure are heat bonded by the wet heat bonding fibers.
Further, this invention also provides a process for producing a porous fibrous structure, which comprises impregnating a fibrous structure containing from 10 to 100% by weight of wet heat bonding fibers with water, then heating the water-containing fibrous structure to evolve bubbles by boiling water in the fibrous structure, and forming a plurality of indefinite cavities in the inside of the fibrous structure and, simultaneously, heat bonding at least a portion of the fibers constituting the fibrous structure by the wet heat bonding fibers.
The wet heat bonding fibers contained in the porous fibrous structure according to this invention are fibers containing a polymer which is softened in hot water at about 95 to 100xc2x0 C. and bond to each other or bond to other fibers.
An example of such polymer can include an ethylene-vinyl alcohol copolymer. The ethylene-vinyl alcohol copolymer means a copolymer in which ethylene residues are copolymerized by 10 to 60 mol% to vinyl alcohol residues. The copolymer copolymerized with 30 to 50 mol% of the ethylene residues are preferred in view of the wet heat bonding property. Further, the vinyl alcohol moiety preferably has a saponification degree of 95 mol% or more. The large content of the ethylene residue can provide a specific nature of not soluble in hot water while having wet heat bonding property. The degree of polymerization can be selected optionally and it is usually about from 400 to 1500. After preparing an aimed porous fibrous structure, the ethylene-vinyl alcohol copolymer can be partially crosslinked for post fabrication such as provision of dyeability and modification of fibers.
Other polymers showing the wet heat bonding property can include, for example, a copolymer containing acryloamide and polylactic acid.
The wet heat bonding fiber may comprise the copolymer described above alone, or may conjugated fibers with other thermoplastic polymer or fibers formed by coating the copolymer to fibers comprising other thermoplastic polymer. It is necessary that the other thermoplastic polymer has higher melting point than the ethylene-vinyl alcohol copolymer in view of the heat resistance and dimension stability, and a thermoplastic polymer having a melting point of 150xc2x0 C. or higher is preferred and it can include, specifically, polyester, polyamide and polypropylene.
The polyester can include those fibers-forming polyesters comprising aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, naphthalene-2,6 -dicarboxylic acid, phthalic acid, xcex1,xcex2-(4-carboxyphenoxy) ethane, 4,4xe2x80x2-dicarboxydiphenyl and 5-sodium sulfoisophthalic acid; aliphatic dicarboxylic acid such as azelaic acid, adipic acid or sebasic acid or esters thereof; and diols such as ethylene glycol, diethylene glycol, 1,3-propane diol, 1,4-butane diol, 1,6-hexane diol, neopentyl glycol, cyclohexane-1,4-dimethanol, polyethylene glycol and polytetramethylene glycol. It is preferred that 80 mol% or more of the constituent units are ethylene terephthalate unit.
The polyamide can include, for example, aliphatic polyamide and semi-aromatic polyamide containing nylon 6, nylon 66 or nylon 12 and it may be a polyamide containing a smaller amount of a third ingredient.
In a case of conjugated fibers comprising an ethylene-vinyl alcohol copolymer and other thermoplastic polymer, the compositional ratio is preferably as the former to the latter ratio (weight ratio) =10:90 to 90:10, particularly, 30:70 to 70:30 in view of the spinnability. There is no particular restriction for the conjugated fiber so long as it is a known conjugated form, and it can include core-sheath type, eccentric core-sheath type, multi-layered type, side-by-side type, random conjugated type and radial conjugated type providing that the copolymer is exposed at least partially of the fiber surface, preferably, by 50% or more. The cross sectional shape of the fibers may not be restricted to a solid circular cross sectional or modified cross sectional shape, but it may be various cross sectional shapes such as macaroni shape.
In a case of using them as a cleaning appliance for human body, cosmetic appliance and cleaning appliance for precision apparatus or-noble metal products, it is preferred that split type conjugated fibers are used as the composite fibers, to form a porous fibrous structure comprising single fibers of 0.1 denier or less and, preferably, 0.01 denier or less, in view of feeling and wiping performance.
Further, in the fibers formed by coating the ethylene-vinyl alcohol copolymer to other thermoplastic fibers, it is preferred that the copolymers covers the surface of other fibers by ⅓ or more and, preferably, xc2xd or more.
It is necessary that the porous fibrous structure of this invention contains the wet heat bonding fibers by 10 to 100% by weight, preferably, 30 to 100% by weight and, further preferably, 50 to 100% by weight. If the wet heat bonding fibers are less than 10% by weight, adhesion of the fibers is insufficient to make the formation of the cellular cavities. The porous fibrous structure is not particularly restricted to woven or knitted products, non-woven fabrics, block form fibrous structures and composite structures thereof, and wadding type or shaped fibrous structures and various kind of needle-punched non-woven fabrics are included. Further, other non-woven fabrics, cloths, films and network-like products may be laminated or sandwiched therewith.
Further, the porous fibrous structure is not restricted to those of plane structure but it may be formed into arbitrary three dimensional indefinite shapes such as of rectangular boxes, cylinders, spheres, dolls or animals. For example, three dimensional porous fibrous structure manufactured by blowing fibers into a shaping mold formed into the shape described above can also be used.
On the other hand, other fibers than the wet heat bonding fibers constituting the porous fibrous structure are not particularly restricted but natural fibers, semi-synthetic fibers and synthetic fibers can be used and they are selected depending on the purposes of application.
The cellular cavities formed in the porous fibrous structure according to this invention have various indefinite shapes including spherical shapes and cloud-type shapes, and are hollow cavities having a size capable of being distinguished clearly from the intra-fiber spaces of the fibers constituting the porous fibrous structure. The size of major axis of the cellular cavities distributes widely from about 1 mm to about 30 mm. The cellular cavities are present in the porous fibrous structure independently or in a state where a plurality of them are connected partially. The independent or continuous shape of the cellular cavities is not restricted particularly but of such a shape as visually observed in a specimen or an enlarged photograph thereof. With a micro point of view, a cellular cavity contiguous for several tens centimeters may exist.
The porous fibrous structure according to this invention has a feature in having a plurality of large cavities with a major diameter of about 5 mm or more. The porosity can be determined optionally depending on the amount of the wet heat bonding fibers, the accumulation density of the fibers and the wet heat treating conditions and it is preferably about 80% or more and, further preferably, 90% or more of the porosity according to the mercury porosimetery.
The cavities are formed without using the foaming agent at all, and have a structure not found in the conventional porous fibrous structures. Referring to the cavities in this invention, the fibrous structure containing wet heat bonding fibers is impregnated with water, the water-containing fibrous structure is heat treated at about 100xc2x0 C., that is, at a boiling point of water to form a great amount of bubbles in the fibrous structure, the fibers of the structure are moved by the bubbles, the resultant spaces form cellular cavities in the fibrous structure, the wet heat bonding fibers are melted simultaneously by the heat to fix the inner wall surface of the cavities, and fibers in other portions are bonded to each other to form an entangled structure.
According to this invention, formation of the cellular cavities and heat bonding of the fibers are taken place simultaneously by the combination of the wet heat bonding fibers and heating with boiling water to form a porous fibrous structure.
The porous fibrous structure according to this invention can be formed as a homogeneous porous fibrous structure having the cellular cavities entirely and also formed as a so-called asymmetric structure having a dense layer on one surface thereof and a porous fibrous layer in contiguous therewith. It is of course possible to use known adhesive or blowing agent for a secondary purpose.